A number of experimental studies have been performed on an x-ray source called the "gas puff Z-pinch" source. This device was first discussed by J. Shiloh et al. in Physical Review Letters, Vol. 40, No. 8, pp. 515-518 (1978). Subsequent versions have been described by C. Stallings et al. in Applied Physics Letters, Vol. 35, No. 7, pp. 524-526 (1979) and by J. S. Pearlman et al. in Journal of Vacuum Science and Technology, Vol. 19, No. 4, pp. 1190-1193 (1981). One form of this device is disclosed in U.S. Pat. No. 4,635,282 issued Jan. 6, 1987 to Okada et al.
The gas puff Z-pinch source involves the introduction of a "gas puff" into a vacuum chamber through an annular orifice. The annular orifice causes the gas puff to form a roughly cylindrical shell within the vacuum chamber. A high current pulse ionizes the gas and produces a plasma shell. The magnetic field associated with the high current causes the plasma shell to collapse toward the axis of the device. The collapsed plasma shell generates x-rays along the device axis. This device has a number of problems and disadvantages which render it impractical for commercial application.
In prior art systems, the driving current pulse has been of much longer duration than the time taken for movement of the plasma shell to the axis of the device. This has meant that the current continued to flow through the plasma in an axial direction, delivering a concentrated flux of ions and electrons onto the nearby part of the electrode structure and causing rapid electrode erosion at this location. The erosion involves the evaporation of metal, which can be deposited on the x-ray output window and decrease its transmission. Also, the erosion can form a particle beam in the direction of the x-ray exit, necessitating elaborate particle removal mechanisms as described, for example, in U.S. Pat. No. 4,837,794 issued Jun. 6, 1989 to Riordan et al.
The passage of current through an axial plasma, while heating the plasma as desired for x-ray production, also causes plasma instabilities to develop, with the result that x-rays are produced from a rapidly moving sequence of hot spots rather than from a single location, as discussed by P. Choi et al. in Review of Scientific Instruments, Vol. 57, p. 2162 (1986). This lowers the usefulness of the source for purposes such as microscopy and lithography, for which stable source position is required.
A further disadvantage of the gas puff Z-pinch source is its requirement for a gas release mechanism, which has been mechanical in all known prior art implementations, and carries with it the failure modes associated with the wear and fatigue of moving mechanisms. The gas is injected into the device principally in order to provide an approximately cylindrical starting shell of gas for the magnetic acceleration process. The device conducts current preferentially through the gas shell when a voltage is applied between its electrodes and, hence, a cylindrical plasma shell is formed. In these devices, the plasma shell may be non-uniform and asymmetrical about the axis.
In all known prior art Z-pinch plasma systems, the high current which drives the plasma acceleration has been switched using high pressure spark gaps. This type of switch has very limited life expectancy (10.sup.5 pulses) because of electrode pitting and metal evaporation which coats the switch insulator. For the application of x-rays to semiconductor lithography, up to 10.sup.6 x-ray pulses per day must be generated without frequent servicing of the switches.
The gas puff creates a density gradient in the direction away from the pinch electrode at which the gas is released. When current is passed through this gas cloud in an axial direction, the heavier parts of it are accelerated more slowly, with the result that they reach the axis later than the lighter parts. This creates a moving x-ray source spread out in time over several tens of nanoseconds. The source peak intensity is therefore degraded.
The advantages of preionization using an electron beam in a small scale Z-pinch x-ray source are described by I. Weinberg et al. in Nuclear Instruments and Methods in Physics Research, Vol. A242, pp. 535-538 (1986). A method for preionizing a static gas cylinder is described by W. Hartmann et al. in Applied Physics Letters, Vol. 58, No. 23, Jun. 10, 1991, pp. 2619-2621. The disclosed method involves a conical discharge at one end of the cylinder and does not produce uniform preionization.